Bigger Tires (Reprogram the Computer?)

[shelbytt]

I just bought :eek: a 2004 GMC Sierra Duramax SLT 4x4 Shortbed, and couldnt stand the small factory 245's on the truck so I just upgraded to some 285X75X16 BFG AT and new 8.5 Chrome Rims.

I was told I need to take it back to the Service Center to get them to reprogram the computer to show the differnt size tires. Is this true? I was also told instead of taking into the Service Center and spending $50-$100 to have them adjust the computer to buy my own programmer as some will do this feature. Can anyone put some feedback here and if so types of Programmers that will do this?

[DmaxMaverick]

The dealer is not able to program the PCM for tire size changes. They can change it to calibrate for the ABS, but not the speedo/odo. They even have policy about denying warranty service for powertrain issues with oversize tires.

My truck has 285/75/16's on 16 X 8's. I have a Predator for just this purpose. It is infinitely adjustable for tire size, which I think is important. Not just programmable for specific sizes. Even the original programming, with OEM tires/wheels leaves most with an inaccurate speedo/odo. Changing tire sizes then programming the PCM to correct the difference is not straight forward. The further you get from OEM size, the greater the error range. Even if you program for 285/75/16, it may still be off by 2% or more. Mine was. I calibrate it with highway mile markers and GPS. It is dead-on accurate now.

There are a couple of programmers that can adjust for tire size (Juice is not one of them). I can only tell you how the Predator does, as I have not used the others. The Predator (which just dropped in price, BTW) can be adjusted for axle ratio, speed limiter, and tire size. It has 5 power settings, 0, 45, 65, 85, and 120 HP. It stores the original factory programming and can be returned to original programming any time you need to. It is also a diagnostic scanner. It will display over 100 real time parameters, as well as read (in plane english) and clear trouble codes (DTC's, or SES codes). It is very easy to use, and as I understand it, the fastest to program (about a minute, start to finish). Real nice, large, backlit display.

I compared all the products available and the Predator has the most features of any of them. It works as advertised. It is priced a little over the other programmers, but, IMO, well worth it. I'm not sure if it available yet for the 2004's with LLY, but should be soon if not. Worth the wait if it works as well on the LLY. I haven't had to deal with customer service, but I understand it is very good. Free UPS next day air shipping.

Check it out at:
http://www.runninwiththedevil.com/gmdiesel.htm

I don't sell Predator, nor do I profit from any sales. It just works, and I am very pleased with it. Very well designed product. Only complaint I can come up with is the case. It came in that "transparent IMAC blueberry" color. Bleck!

[AlanL]

Even if you program for 285/75/16, it may still be off by 2% or more. Mine was. I calibrate it with highway mile markers and GPS. It is dead-on accurate now. Your speedo is *never* going to be dead-on accurate... for more than a few minutes, anyway. As you drive, your tires heat up and cool down changing their diameter/circumference slightly because of changes in air pressure related to heat.

They also continually wear down, changing the size as well. With a tread depth of 13/16 (about 11/16 of which is usable before the tires are considered worn out) there is 1.375 inches of difference in the diameter/circumference when comparing a new tire to a worn tire.

So, your speedo was dead-on for the day you set it, at the speed you set it, at the ambient and pavement temperatures at which you set it, with the tires worn exactly where they were when you set it. :rolleyes:

IMO the best thing to do would be for the factory (or the predator) to use a factor that would be for a half worn tire. In doing this, the speedo would be off by its greatest amount only when the tires were new and also when they're completely worn. As new tires wore down, the speedo would get more and more acurate until the tires were 1/2 worn, at which time the error would grow again to a maximum, but in the other direction.

I believe, however, that for legal reasons (the B.S. reason for so many things these days) auto manufacturers have their speedos error on the high side so that people are less likely to be able to bring suits against them with regard to speeding such as saying "I got this speeding ticket because my speedo was registering low".

Given all of this, 2% seems like a good compromise to me. ;)

[Heartbeat Hauler]

DmaxMaverick,
Using the Predator, can you do the tire calibrations only without loading the performance upgrades?
I have been trying to talk a vendor into offering a tuner that only does the basics, i.e. Tire/gear change calibrations, speed limiter removal, and code reading and clearing. One vendor was sorta interested, but I haven't heard anything for a few weeks now. Anyways, your info would help.
Thanks,
JP

[AlanL]

Yes, you can do a tire calibration change without having a performance enhancement program loaded.

[DmaxMaverick]

Heartbeat Hauler

Yes. Each "power setting" has individually programmable mod's. Just modify the tire size and install the "factory tune". It will not alter the factory power level. All other features are also available. The scanner feature works on several other vehicles as well. Although it will only allow DTC reading/clearing on the vehicle it is "married" to, all the real time data is readable. I tried it on several 00 to 03 1500's, hd's and even a 00 Impala. Some of the data is "generic" on non-Duramax vehicles, but translates easily.


Alan

Gimme a break. :rolleyes: Dead-on accurate. Absolutely. Yes, the tires will wear, conditions will change, etc. We are dealing with analog speedo's, so accuracy is "perceived". If it were digital, it would be a different story. When the tires wear enough to throw it off, I'll reprogram it. Only takes a minute. As far as temp and air pressure goes, I've never seen any change because of it, empty or loaded. Any change would be more slight than I can detect. This is not rocket science (but pretty close). The circumference, or roll out of the tire won't change (except for wear), regardless of pressure, temp, etc. by any detectable amount unless you measure a mile with a micrometer. If they do, you should be a little more selective about the quality of tire you are using.


I believe, however, that for legal reasons (the B.S. reason for so many things these days) auto manufacturers have their speedos error on the high side so that people are less likely to be able to bring suits against them with regard to speeding such as saying "I got this speeding ticket because my speedo was registering low". Is this based in fact? If so, where? Sounds like a manufacturer's excuse as to why the instruments are so inaccurate. What about the fuel gage, or oil pressure, or voltmeter???? On which side do they err? If I could calibrate them as easily, I would.

Why should I be off by 2% if I don't have to (or want to)? "Compromise" if you must.

Cheers

[AlanL]

I'm not trying to bust anyone's chops here :(

I often see people write things in the forum, however, that talk about tires and such like they're absolutes. The tires on your vehicle are ever changing because of wear and inflation... whether its a cheap tire or an expensive tire. For instance 10 degrees of ambient temperature change is equal to 1 psi difference in tire pressure. That's a fact and it effects your tires.

With regard to the speedos, I've read things, over time about them being off, and usually biased high. I'm not sure exactly where I read it or in what magazine (I get or have gotten lots of car magazines for the past 20 years). The excuse given (and I agree that its an excuse) is that by having them read high they help protect themselves against suits regarding speeding or accidents where speeding was deemed to be a contributing factor.

The 2% doesn't seem like a big deal to me since I'm going the speed I feel is appropriate anyway, regardless of gauges. I bough the predator to adjust for the 265 tires on my truck because without it, the error would be about 6% which seemed excessive.

[mdadgar]

Alan, you read it in Car & Driver about a year ago. They had an article about optimistic speedos in new cars and how the manufacturers bias them that way, particularly on cars that are likely to get wheel/tire upgrades by the owners (ie BMW, Mercedes, Porsche, etc).

I recently had my dealer recalibrate my ABS for 265 tires. The dealer SWEARS that they recalibrated the speedo, too. They even said they had to call GM for a new VCI number. I haven't had time to test it yet. We'll see.

- Mark